Functions in Elixir have both a name AND an arity (number of arguments it takes). For Elixir to find the function, it needs both. Therefore, if you use the & capture operator, you also need to specify the function’s arity:

Enum.all?([1, 30], &is_number/1)

With your previous syntax of Enum.all?([1, 30], is_number), Elixir doesn’t know the function’s arity. In this case, there is only one version of is_number, but other functions with the same name can have different arities, e.g. List.flatten/1 and List.flatten/2.

In Elixir, a function’s “identity” is composed of BOTH its name and arity. This is why you’ll see functions referred to as is_number/1 and not just is_number.